Such hydrates may form when water is in the presence of light hydrocarbons, either in the gas phase or dissolved in a liquid phase, such as a liquid hydrocarbon, and when the temperature reached by the mixture becomes lower than the thermodynamic hydrate formation temperature, this temperature being given for a given gas composition and pressure value.
Hydrate formation can be feared notably in the petroleum and gas industry where hydrate formation conditions can be met. In fact, in order to decrease the production cost of crude oil and gas, as regards investment as well as development, one of the means considered, notably in offshore production, consists in reducing or even doing away with the processings applied to the crude oil or to the gas to be transported from the reservoir to the coast, and notably in keeping all or part of the water present in the fluid to be transported. Offshore processings are generally performed on a platform situated at the surface close to the reservoir, so that the effluent, initially hot, can be processed before the thermodynamic hydrate formation conditions are reached as a result of the cooling of the effluent circulating in a pipe in indirect contact with sea water. This procedure is particularly advantageous when offshore production is performed in a zone that is difficult of access. However, the major drawback thereof lies in the risks of hydrate formation due to the presence of water.
In fact, petroleum effluents containing a gas phase and possibly a liquid phase can consist, for example, of a natural gas, a condensate gas or an associated gas mixed with crude oil. They are generally water-saturated and they may even contain free water in some cases.
During the transportation of production effluents from an underwater natural gas or oil and gas reservoir, containing water, the sea bottom temperature can be of the order of 3.degree. or 4.degree. C. Such a temperature leads to a fall in the temperature of the effluent produced, which can bring the latter into thermodynamic conditions favourable to the formation of hydrates, that agglomerate and block production pipes.
Hydrates are inclusion compounds formed from water and light hydrocarbons such as methane, ethane, propane, iso or n-butane. Nitrogen, as well as certain acid gases present in natural gas, such as carbon dioxide or hydrogen sulfide, can also form hydrates in the presence of water.
The formation, then the agglomeration of hydrates lead to the filling and to the clogging of transmission pipes, which eventually prevents passage of oil or gas and has extremely serious consequences. In fact, these phenomena can lead to production stops and to considerable financial losses. Furthermore, restarting the facility, notably in the case of offshore production or transportation, can be long since the decomposition of the hydrates formed is very difficult to achieve.
Conditions favourable to the formation of hydrates can also be met in the same way onshore, for pipes buried near the ground surface for example when the temperature of the ambient air is quite low, notably in northern zones such as arctic zones.
In order to avoid such drawbacks, one has tried in the prior art to use products that, added to the fluid, act as inhibitors by lowering the thermodynamic hydrate formation temperature. These are notably alcohols, such as methanol, or glycols, such as mono-, di- or triethylene glycol. This solution is very costly since the amount of inhibitors to be added can reach 10 to 50% by weight of the water content and these inhibitors are difficult to recover completely.
It has also been recommended to insulate or even to heat transmission pipes by means of a suitable device, such as described in patent application WO-90/05,260, in order to prevent too fast a cooling of the fluids transported. Such devices are nevertheless expensive and complex as far as their technical realization is concerned.
Other means consist in using radiations, for example, patent HU-18,511 teaches to send an electromagnetic wave whose frequency values and modes of propagation are selected to cause the hydrates formed to melt.
In patent SU-442,287, an ultrasonic wave is used to break the hydrate crystals and to release the trapped gas.
The use of additives capable of modifying the hydrate formation mechanism has also been described, where the hydrates formed disperse in the fluid without agglomerating and without clogging the pipes rather than quickly agglomerate with each other and form very solid plugs. The applicant's patent application EP-A-323,774 can be cited in this respect, which describes the use of non-ionic amphiphile compounds selected from the polyol and carboxylic acid esters, substituted or non-substituted, and the imide function compounds. The applicant's patent application EP-A-323,775 can also be cited, which notably describes the use of compounds belonging to the fatty acid or fatty acid derivative diethanolamides family. Patent U.S. Pat. No. 4,956,593 describes the use of surface-active compounds such as organic phosphonates, phosphate esters, phosphonic acids, their salts and their esters, organic polyphosphates and their esters, as well as polyacrylates and polyacrylamides. Patent application EP-A,457,375 describes the use of anionic surface-active compounds such as alkylsulfonic acids and their alkali metal salts. Amphiphile compounds obtained by reaction of at least one succinic derivative, selected from the group consisting of the amides and the polyalkenylsuccinic anhydrides, on at least one polyethyleneglycol monoether have also been proposed to reduce the agglomeration tendency of gas hydrates in patent application EP-A-582,507.
Methods advocating the use of dispersing chemical additives for the transportation of hydrates are interesting because of the low amounts of additives necessary to disperse the hydrate crystals in the liquid hydrocarbon phase. However, the resulting economic saving is limited on the grounds of the cost of the additive, that is higher than the cost of conventional inhibitors such as methanol or the glycols.
It is therefore advantageous and beneficial to recover and to re-use the dispersing additive at least partly.